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      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Rule</b></font>
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<p>The <b>rule</b> is a polymorphic parser that acts as a named place-holder capturing 
  the behavior of an EBNF expression assigned to it. Naming an EBNF expression 
  allows it to be referenced later. The <tt>rule</tt> is a template class parameterized 
  by the type of the scanner (<tt>ScannerT</tt>), the rule's <a href="indepth_the_parser_context.html">context</a> 
  and its <a href="#tag">tag</a>. Default template parameters are provided to 
  make it easy to use the rule.</p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>template</span><span class=special>&lt;
        </span><span class=keyword>typename </span><span class=identifier>ScannerT </span><span class=special>= </span><span class=identifier>scanner</span><span class=special>&lt;&gt;,
        </span><span class=keyword>typename </span><span class=identifier>ContextT </span><span class=special>= </span><span class=identifier>parser_context</span><span class=special>&lt;&gt;</span><span class=identifier>,
        </span><span class="keyword">typename</span><span class=identifier> TagT </span><span class="special">=</span><span class=identifier> parser_address_tag</span><span class=special>&gt;
    </span><span class=keyword>class </span><span class=identifier>rule</span><span class=special>;</span></font></code></pre>
<p>Default template parameters are supplied to handle the most common case. <tt>ScannerT</tt> 
  defaults to <tt>scanner&lt;&gt;</tt>, a plain vanilla scanner that acts on <tt>char 
  const<span class="operators">*</span></tt> iterators and does nothing special 
  at all other than iterate through all the chars in the null terminated input 
  a character at a time. The rule tag, <tt>TagT</tt>, typically used with <a href="trees.html">ASTs</a>, 
  is used to identify a rule; it is explained <a href="#tag">here</a>. In trivial 
  cases, declaring a rule as <tt>rule&lt;&gt;</tt> is enough. You need not be 
  concerned at all with the <tt>ContextT</tt> template parameter unless you wish 
  to tweak the low level behavior of the rule. Detailed information on the <tt>ContextT</tt> 
  template parameter is provided <a href="indepth_the_parser_context.html">elsewhere</a>. 
</p>
<h3><a name="order_of_parameters"></a>Order of parameters</h3>
<p>As of v1.8.0, the <tt>ScannerT</tt>, <tt>ContextT</tt> and <tt>TagT</tt> can 
  be specified in any order. If a template parameter is missing, it will assume 
  the defaults. Examples:</p>
<pre><span class=identifier>    rule</span><span class=special>&lt;&gt; </span><span class=identifier>rx1</span><span class=special>;
    </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>scanner</span><span class=special>&lt;&gt; </span><span class=special>&gt; </span><span class=identifier>rx2</span><span class=special>;
</span>    <span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>parser_context<code><font color="#000000"><span class=special>&lt;&gt;</span></font></code> </span><span class=special>&gt; </span><span class=identifier>rx3</span><span class=special>;
    </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>parser_context<code><font color="#000000"><span class=special>&lt;&gt;</span></font></code></span><span class=special>, </span><span class=identifier>parser_address_tag</span><span class=special>&gt; </span><span class=identifier>rx4</span><span class=special>;
</span>    <span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>parser_address_tag</span><span class=special>&gt; </span><span class=identifier>rx5</span><span class=special>;
</span>    <span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>parser_address_tag</span><span class=special>, </span><span class=identifier>scanner</span><span class=special>&lt;&gt;, </span><span class=identifier>parser_context<code><font color="#000000"><span class=special>&lt;&gt;</span></font></code> </span><span class=special>&gt; </span><span class=identifier>rx6</span><span class=special>;
    </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>parser_context<code><font color="#000000"><span class=special>&lt;&gt;</span></font></code></span><span class=special>, </span><span class=identifier>scanner</span><span class=special>&lt;&gt;, </span><span class=identifier>parser_address_tag</span><span class=special>&gt; </span><span class=identifier>rx7</span><span class=special>;</span></pre>
<h3><a name="multiple_scanner_support" id="multiple_scanner_support"></a>Multiple scanners</h3>
<p>As of v1.8.0, rules can use one or more scanner types. There are cases, for 
  instance, where we need a rule that can work on the phrase and character levels. 
  Rule/scanner mismatch has been a source of confusion and is the no. 1 <a href="faq.html#scanner_business">FAQ</a>. 
  To address this issue, we now have multiple scanner support. Example:</p>
<pre><span class=special>    </span><span class=keyword>typedef </span><span class=identifier>scanner_list</span><span class=special>&lt;</span><span class=identifier>scanner</span><span class=special>&lt;&gt;, </span><span class=identifier>phrase_scanner_t</span><span class=special>&gt; </span><span class=identifier>scanners</span><span class=special>;

    </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>scanners</span><span class=special>&gt;  </span><span class=identifier>r </span><span class=special>= </span><span class=special>+</span><span class=identifier>anychar_p</span><span class=special>;
    </span><span class=identifier>assert</span><span class=special>(</span><span class=identifier>parse</span><span class=special>(</span><span class=string>&quot;abcdefghijk&quot;</span><span class=special>, </span><span class=identifier>r</span><span class=special>).</span><span class=identifier>full</span><span class=special>);
    </span><span class=identifier>assert</span><span class=special>(</span><span class=identifier>parse</span><span class=special>(</span><span class=string>&quot;a b c d e f g h i j k&quot;</span><span class=special>, </span><span class=identifier>r</span><span class=special>, </span><span class=identifier>space_p</span><span class=special>).</span><span class=identifier>full</span><span class=special>);</span></pre>
<p>Notice how rule <tt>r</tt> is used in both the phrase and character levels. 
</p>
<p>By default support for multiple scanners is disabled.  The macro
  <tt>BOOST_SPIRIT_RULE_SCANNERTYPE_LIMIT</tt> must be defined to the
  maximum number of scanners allowed in a scanner_list.  The value must
  be greater than 1 to enable multiple scanners.  Given the
  example above, to define a limit of two scanners for the list, the
  following line must be inserted into the source file before the
  inclusion of Spirit headers:
</p>
<pre><span class=special>    </span><span class=preprocessor>#define </span><span class=identifier>BOOST_SPIRIT_RULE_SCANNERTYPE_LIMIT</span> <span class=literal>2</span></pre>
<table width="80%" border="0" align="center">
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    <td class="note_box"><img src="theme/bulb.gif" width="13" height="18"> See 
      the techniques section for an <a href="techniques.html#multiple_scanner_support">example</a> 
      of a <a href="grammar.html">grammar</a> using a multiple scanner enabled 
      rule, <a href="scanner.html#lexeme_scanner">lexeme_scanner</a> and <a href="scanner.html#as_lower_scanner">as_lower_scanner.</a></td>
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<h3>Rule Declarations</h3>
<p>The rule class models EBNF's production rule. Example:</p>
<pre><code><font color="#000000">    <span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>a_rule </span><span class=special>= </span><span class=special>*(</span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b</span><span class=special>) </span><span class=special>& </span><span class=special>+(</span><span class=identifier>c </span><span class=special>| </span><span class=identifier>d </span><span class=special>| </span><span class=identifier>e</span><span class=special>);</span></font></code></pre>
<p>The type and behavior of the right-hand (rhs) EBNF expression, which may be 
  arbitrarily complex, is encoded in the rule named a_rule. a_rule may now be 
  referenced elsewhere in the grammar:</p>
<pre><code><font color="#000000">    <span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>another_rule </span><span class=special>= </span><span class=identifier>f </span><span class=special>&gt;&gt; </span><span class=identifier>g </span><span class=special>&gt;&gt; </span><span class=identifier>h </span><span class=special>&gt;&gt; </span><span class=identifier>a_rule</span><span class=special>;</span></font></code></pre>
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    <td class="note_box"><img src="theme/alert.gif" width="16" height="16"> <b>Referencing 
      rules <br>
      </b><br>
      When a rule is referenced anywhere in the right hand side of an EBNF expression, 
      the rule is held by the expression by reference. It is the responsibility 
      of the client to ensure that the referenced rule stays in scope and does 
      not get destructed while it is being referenced. </td>
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<pre><span class=special>    </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>int_p</span><span class=special>;
    </span><span class=identifier>b </span><span class=special>= </span><span class=identifier>a</span><span class=special>;
    </span><span class=identifier>c </span><span class=special>= </span><span class=identifier>int_p </span><span class=special>&gt;&gt; </span><span class=identifier>b</span><span class=special>;</span></pre>
<h3>Copying Rules</h3>
<p>The rule is a weird C++ citizen, unlike any other C++ object. It does not have 
  the proper copy and assignment semantics and cannot be stored and passed around 
  by value. If you need to copy a rule you have to explicitly call its member 
  function <tt>copy()</tt>:</p>
<pre><span class=special>    </span><span class=identifier>r</span><span class="special">.</span><span class=identifier>copy()</span><span class=special>;</span></pre>
<p>However, be warned that copying a rule will not deep copy other referenced 
  rules of the source rule being copied. This might lead to dangling references. 
  Again, it is the responsibility of the client to ensure that all referenced 
  rules stay in scope and does not get destructed while it is being referenced. 
  Caveat emptor.</p>
<p>If you copy a rule, then you'll want to place it in a storage somewhere. The 
  problem is how? The storage can't be another rule:</p>
<pre>    <code><font color="#000000"><span class=identifier>rule</span><span class=special>&lt;&gt;</span></font></code> r2 <span class="special">=</span> <span class=identifier>r</span><span class="special">.</span><span class=identifier>copy()</span><span class=special>; </span><span class="comment">// BAD!</span></pre>
<p>because rules are weird and does not have the expected C++ copy-constructor 
  and assignment semantics! As a general rule: <strong>Don't put a copied rule 
  into another rule! </strong>Instead, use the <a href="stored_rule.html">stored_rule</a> 
  for that purpose.</p>
<h3>Forward declarations</h3>
<p>A <tt>rule</tt> may be declared before being defined to allow cyclic structures 
  typically found in BNF declarations. Example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>a</span><span class=special>, </span><span class=identifier>b</span><span class=special>, </span><span class=identifier>c</span><span class=special>;

    </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>b </span><span class=special>| </span><span class=identifier>a</span><span class=special>;
    </span><span class=identifier>b </span><span class=special>= </span><span class=identifier>c </span><span class=special>| </span><span class=identifier>a</span><span class=special>;</span></font></code></pre>
<h3>Recursion</h3>
<p>The right-hand side of a rule may reference other rules, including itself. 
  The limitation is that direct or indirect left recursion is not allowed (this 
  is an unchecked run-time error that results in an infinite loop). This is typical 
  of top-down parsers. Example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b</span><span class=special>; </span><span class=comment>// infinite loop!</span></font></code></pre>
<table width="80%" border="0" align="center">
  <tr> 
    <td class="note_box"><img src="theme/lens.gif" width="15" height="16"> <b>What 
      is left recursion?<br>
      </b><br>
      Left recursion happens when you have a rule that calls itself before anything 
      else. A top-down parser will go into an infinite loop when this happens. 
      See the <a href="faq.html#left_recursion">FAQ</a> for details on how to 
      eliminate left recursion.</td>
  </tr>
</table>
<h3>Undefined rules</h3>
<p>An undefined rule matches nothing and is semantically equivalent to <tt>nothing_p</tt>.</p>
<h3>Redeclarations</h3>
<p>Like any other C++ assignment, a second assignment to a rule is destructive 
  and will redefine it. The old definition is lost. Rules are dynamic. A rule 
  can change its definition anytime:</p>
<pre><code><font color="#000000"><span class=identifier>    r </span><span class=special>= </span><span class=identifier>a_definition</span><span class=special>;
</span><span class=identifier>    r </span><span class=special>= </span><span class=identifier>another_definition</span><span class=special>;</span></font></code></pre>
<p>Rule <tt>r</tt> loses the old definition when the second assignment is made. 
  As mentioned, an undefined rule matches nothing and is semantically equivalent 
  to <tt>nothing_p</tt>. 
<h3>Dynamic Parsers</h3>
<p>Hosting declarative EBNF in imperative C++ yields an interesting blend. We 
  have the best of both worlds. We have the ability to conveniently modify the 
  grammar at run time using imperative constructs such as <tt>if</tt>, <tt>else</tt> 
  statements. Example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>if </span><span class=special>(</span><span class=identifier>feature_is_available</span><span class=special>)
        </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>add_this_feature</span><span class=special>;</span></font></code></pre>
<p>Rules are essentially dynamic parsers. A dynamic parser is characterized by 
  its ability to modify its behavior at run time. Initially, an undefined rule 
  matches nothing. At any time, the rule may be defined and redefined, thus, dynamically 
  altering its behavior.</p>
<h3>No start rule</h3>
<p>Typically, parsers have what is called a start symbol, chosen to be the root 
  of the grammar where parsing starts. The Spirit parser framework has no notion 
  of a start symbol. Any rule can be a start symbol. This feature promotes step-wise 
  creation of parsers. We can build parsers from the bottom up while fully testing 
  each level or module up until we get to the top-most level.</p>
<h3><a name="tag"></a>Parser Tags</h3>
<p>Rules may be tagged for identification purposes. This is necessary, especially 
  when dealing with <a href="trees.html">parse trees and ASTs</a> to see which 
  rule created a specific AST/parse tree node. Each rule has an ID of type <tt>parser_id</tt>. 
  This ID can be obtained through the rule's <tt>id()</tt> member function:</p>
<pre><code><font color="#000000"><span class=identifier>    my_rule</span><span class=special>.</span><span class=identifier>id</span><span class=special>(); </span><span class=comment>//  get my_rule's id</span></font></code></pre>
<p>The <tt>parser_id</tt> class is declared as:</p>
<pre>    <span class="keyword">class</span> <span class="identifier">parser_id</span><br>    <span class="special">{</span><br>    <span class="keyword">public</span><span class="special">:</span><br>                    parser_id<span class="special">();</span><br>        <span class="keyword">explicit</span>    parser_id<span class="special">(</span><span class="keyword">void const</span><span class="special">*</span> p<span class="special">);</span><br>                    parser_id<span class="special">(</span><span class="keyword">std::size_t</span> l<span class="special">);</span>
    
        <span class="keyword">bool</span>        <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">parser_id</span> <span class="keyword">const</span><span class="special">&amp;</span> x<span class="special">)</span> const<span class="special">;</span><br>        <span class="keyword">bool</span>        <span class="keyword">operator</span><span class="special">!=(</span><span class="identifier">parser_id</span> <span class="keyword">const</span><span class="special">&amp;</span> x<span class="special">)</span> const<span class="special">;</span>
        <span class="keyword">bool</span> <span class="keyword">       operator</span><span class="special">&lt;(</span><span class="identifier">parser_id</span> <span class="keyword">const</span><span class="special">&amp;</span> x<span class="special">)</span> const<span class="special">;</span>
        <span class="special"></span><span class="keyword">std::size_t</span><span class="identifier"> to_long</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;
    };</span></pre>
<h3>parser_address_tag</h3>
<p>The rule's <tt>TagT</tt> template parameter supplies this ID. This defaults 
  to <tt>parser_address_tag</tt>. The <tt>parser_address_tag</tt> uses the address 
  of the rule as its ID. This is often not the most convenient, since it is not 
  always possible to get the address of a rule to compare against. </p>
<h3>parser_tag</h3>
<p>It is possible to have specific constant integers to identify a rule. For this 
  purpose, we can use the <tt>parser_tag&lt;N&gt;</tt>, where N is a constant 
  integer:</p>
<pre><code><font color="#000000"><span class=identifier>    rule</span><span class=special>&lt;</span><span class=identifier>parser_tag</span><span class="special">&lt;</span><span class=identifier>123</span><span class="special">&gt; &gt; </span><span class="identifier">my_rule</span><span class="special">; </span><span class="comment">//  set my_rule's id to 123</span></font></code></pre>
<h3>dynamic_parser_tag</h3>
<p>The <tt>parser_tag&lt;N&gt;</tt> can only specify a <strong>static ID</strong>, 
  which is defined at compile time. If you need the ID to be <strong>dynamic</strong> 
  (changeable at runtime), you can use the <tt>dynamic_parser_tag</tt> class as 
  the <tt>TagT</tt> template parameter. This template parameter enables the <tt>set_id()</tt> 
  function, which may be used to set the required id at runtime:</p>
<pre><code><font color="#000000"><span class=identifier>    rule</span><span class=special>&lt;</span><span class=identifier>dynamic_parser_tag</span><span class="special">&gt; </span><span class="identifier">my_dynrule</span><span class="special">;</span>
    my_dynrule.set_id(1234);    <span class="comment">// set my_dynrule's id to 1234</span></font></code></pre>
<p>If the <tt>set_id()</tt> function isn't called, the parser id defaults to the 
  address of the rule as its ID, just like the <tt>parser_address_tag</tt> template 
  parameter would do. </p>
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